Serving on a nonprofit board is another good option. That's something I started this year, along with writing fiction, that has been very gratifying. It may also allow you to use skills that are underutilized at work.
I've also done quite a bit of hands-on, direct volunteer work, but in my experience, that only makes my work dissatisfaction that much more intense. It makes my time at work feel that much more pointless and ridiculous. That's not an argument against volunteering--just a suggestion that volunteering outside of work might not make twiddling your thumbs at work any easier.
I really like the recommendation to serve local government. My city has a formal program for training/developing citizens who want to get more involved in their government. There is an application process to identify good candidates and ensure representation from across the city. You might want to look for something like that in your area. I could also see that leading to more interesting paid work in the long run.
In terms of therapy, the best thing I've found is writing. I didn't really appreciate the term "creative outlet" until I started writing. Doing something creative allows me to release all that energy and drive that I have to keep pent up at work everyday. At this point, I don't know what I would do without it.
